This is a research study to compare the degree of pain relief (analgesia) from the drug neostigmine when injected into the back near the spine (spinal) in men and in women. Neostigmine is a non-narcotic medicine which is approved for injection in the vein, but not spinally. We and others have been studying neostigmine for spinal injection for 5 years, both in volunteers and in patients, and have found it to be an effective pain reliever. In our first studies, it was noted that neostigmine's affect appeared greater in women than in men. In order to confirm this sex difference and to see if it is due to estrogen exposure, 20 men and 40 women (20 during estrogen withdrawal time of oral contraceptives and 20 during estrogen treatment) will be enrolled to receive intrathecal neostigmine and test analgesia to a pain stimulus.